Mobile radio systems today use various mobile radio standards such as Global System for Mobile communication, GSM, Enhanced Data Rates for GSM Evolution, EDGE, Universal Mobile Telecommunications Standard, UMTS, or others. In this case, radiofrequency signals are used for transmission.
Digitally controlled oscillators, DCOs, are increasingly being used to generate and receive the radiofrequency transmission/reception signals. A DCO generates a radiofrequency signal as an output signal on the basis of a digital frequency word. In addition, a digital phase locked loop comprising a DCO needs less space on a semiconductor body than a corresponding phase locked loop comprising an analog-controlled voltage controlled oscillator (VCO).
In mobile radio systems, DCOs operate not only as simple oscillators for generating local oscillator signals, but can also be used for direct modulation of baseband signals which are to be transmitted. FIG. 7 shows an exemplary embodiment of a conventional digitally controlled phase locked loop which is designed for two-point modulation. The phase locked loop comprises a digitally controlled oscillator DCO for generating an oscillator signal RFOut. The oscillator signal RFout is returned to a phase detector PD via a frequency divider MMT which has an adjustable divider ratio. The phase detector PD has a second input for receiving a reference frequency signal at a reference frequency Fref. The output of the phase detector PD is coupled to the input of the digitally controlled oscillator DCO via a digital loop filter LF and a summator S5. The second input of the summator S5 has a modulation device MOD1 connected to it which can be used to supply a modulation setting word Nmod to the phase locked loop. The summator S5 is thus simultaneously a first modulation point MP1 of the phase locked loop.
A control input for setting the divider ratio of the frequency divider MMT forms the second modulation point MP2 of the phase locked loop. This input receives a sequence setting word Nseq which is ascertained by means of a summator S7 from an integer channel setting word Nchan,int and a sigma-delta modulated, fractional rational component. The component is provided by a sigma-delta modulator ΣΔ. The sigma-delta modulator ΣΔ is supplied the modulation setting word Nmod and a fractional channel setting word Nchan,frac via the summator S6.
The modulation data represented by the modulation setting word Nmod are thus supplied to the phase locked loop via the first modulation point MP1 and the second modulation point MP2. In this case, the supply via the second modulation point MP2 has a low-pass filter response, while the supply via the first modulation point MP1 and the first modulation device MOD1 has a high-pass filter response. In this context, it is necessary to know the gradient of the digitally controlled oscillator DCO, since otherwise the modulation signals can be adapted to the response characteristic of the open loop only with difficulty. The gradient of the oscillator DCO is expressed by a gradient factor KDCO. The gradient factor KDCO indicates the effect which a change in the modulation setting word Nmod has on the output frequency of the digitally controlled oscillator DCO, which can be expressed as follows on the basis of a frequency change Δf and a change in the input word Δy for the sigma-delta modulator ΣΔ:
                              K          DCO                =                              Δ            ⁢                                                  ⁢            f                                Δ            ⁢                                                  ⁢            y                                              (        1        )            
If the value of the gradient factor KDCO which is used in the modulation device MOD1 does not correspond to the real gradient of the digitally controlled oscillator DCO, inadmissible distortions in the modulated output signal from the oscillator DCO may arise. This increases the error vector magnitude, EVM.
FIG. 8 shows an example signal/time graph for output signals from a digitally controlled oscillator, where the frequency of the signals is plotted over time. Up to time t1, a first modulation setting word Nmod1 is supplied unchanged. At time t1, there is a switch to a second modulation setting word Nmod2, which is intended to prompt a sudden frequency change ΔF. When the gradient factor KDCO is assumed to be at an optimum, there is a resultant ideal sudden change in the frequency of the output signal from the digitally controlled oscillator DCO by the frequency ΔF, characterized by the solid line KDCO0.
If the gradient factor KDCO is assumed to be too great, the phase locked loop locks more slowly up to the desired frequency of the output signal, characterized by the dashed line KDCO1. If the value of the gradient factor KDCO is assumed to be too small, there is an overshot in the frequency of the output signal and subsequent locking onto the desired frequency from below, characterized by the dashed line KDCO2. The lengthened transient process results in distortions in the output signal, particularly in the case of direct modulation, since the switch between various modulation setting words is sometimes very fast, which means that the desired output signal cannot be generated.
It may therefore be desirable to determine the gradient factor KDCO for modulating the phase locked loop as accurately as possible. The gradient of the digitally controlled oscillator DCO should also be able to be ascertained during operation, since it can change during operation—for example as a result of temperature-dependent drift processes. In the case of modulation methods operating on the basis of the Time Division mode Multiple Access TDMA, method, as in the case of GSM, for example, it is possible to ascertain the oscillator gradient in the transmission breaks between two data bursts, that is to say at times at which the oscillator signal is not primarily required. If, by contrast, a Code Division Multiple Access, CDMA, method is being used, as in the case of UMTS, for example, these breaks in which it is possible to ascertain the oscillator gradient are normally not available. In such systems performing continuous modulation in the phase locked loop, necessary determination of the gradient factor KDCO can be performed during operation only with difficulty and with a high level of complexity.